Keywords: buckling, cylindrical shells, multilayered shells, thermal barrier, shear.

Many aerospace structures such as launchers are equipped with a thermal foam protection barrier. This layer, of extremely light material generally exhibits excellent properties of heat insulation but very weak mechanical characteristics. This study is devoted to the analysis of the buckling behaviour of this hybrid wall construction where the skin of a very thin light-weight structure, an aluminum cylindrical shell with a radius to thickness ratio of about 665, is coated with a thick layer of foam with a low material density. To gauge the effect on the buckling behaviour of this kind of multilayered shells, experiments are conducted on slightly pressurized cylindrical shells submitted to shear load. This load configuration represents the case of a rocket on the launching pad, waiting to be launched, and subjected to mechanical loads induced by the wind.

Several experiments are conducted on scaled models [1,2]. The results permit the assessment of the effect of this foam layer on the critical behaviour associated with the linear bifurcation, and also on the post-critical behaviour. A significant increase in the bearing capacity (first bifurcation) is found at zero pressure or a low pressurization level (less than 400mbars). The foam layer also contributes to inhibit unstable post-critical behaviour. The enhancement of the critical buckling load hence is accompanied with a decrease of the imperfection sensitivity. Numerical analysis is also conducted with the ABAQUS finite element code, an agreement with the experimental results is noticed, confirming that a suitable gain on the bearing capacity can be obtained with this concept of multilayered foam-metallic shells. For the numerical analysis, the classical approach with multilayered shell elements is proposed and gauged comparative to a three-dimensional simulation using solid elements. In the case of a perfect adherence between the two layers, experimental and numerical results obtained using a multilayered shell element approach are in good agreement. Bound imperfections between the two layers are also gauged experimentally and numerically using a simplified and robust approach, where the thermal barrier is inefficient in a local zone. Finally several tests with different radius to thickness ratios and for different materials (metal layer and thermal barrier layer) are conducted and simulated. This permits us to gauge the robustness of the results and to analyse the efficiency of the conducted numerical simulations.

1

A. Da Silva, "Flambage de coques cylindriques minces sous chargements combinés pression interne, compression, flexion et cisaillement", Thèse de Doctorat, INSA de Lyon, 2010.

2

J. Didier, A. Limam, V. Taponier, F. Lorioux, S. Radulovic, "Buckling of pressurized multilayered foam aluminum cylindrical shells submitted to shear load", Aerospace AIRTEC2011 6th International Conference - Supply on the wings, Frankfurt/Main, Germany, November 2011.

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